@geerlingguy, I managed to get AMD Radeon RX 6700 XT working with UEFI version 9.0.0 in ACPI mode and Ubuntu 25.04. However, I had to build my own kernel (I could provide instructions, if necessary) - if I am not mistaken, I applied the patch attached here on top of commit 7a13c14ee59d4f6c5f4277a86516cbc73a1383a8 (that is between tags v6.15-rc4 and v6.15-rc5).
Doom 3 (in the form of dhewm3), GravityMark, and Shadertoy in a browser seem to work fine. Actually, Doom 3 had some visual artifacts, but after I switched the OpenGL driver to Zink (with MESA_LOADER_DRIVER_OVERRIDE=zink as an environment variable) they disappeared. Setting RUSTICL_ENABLE=radeonsi let me run the Geekbench Compute benchmark, but it complained during the result validation at the end and performance was atrocious (more than an order of magnitude lower than expected). The older Clover OpenCL implementation led to a kernel panic.